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Investigation of the Crystallinity The IR-reflectance spectrum of chalcedony, which of the Modifications of Silica by their is a cryptocrystalline modification of silica, is very IR-Reflectance Spectra similar to that of quartz. Both of them show five peaks at or near 1176,1108, 800, 780 and 690 cm"1 with the A. Banerjee difference that the peaks at 780 and 800 cm-1 for Institut für Geowissenschaften - Edelsteinforschung, chalcedony are not as sharp as in the case of quartz. Johannes-Gutenberg-Universität Mainz, Mainz In addition to that, an additional peak near 537 cm-1 Z. Naturforsch. 48a, 741-742 (1993); is found to occur in case of chalcedony. Similar differ- received March 26, 1993 ences were also found between the spectra of chalce- dony and jasper, a microcrystalline modification of Infrared reflectance spectroscopy has been used for the identification of the different modifications of silica and for silica. Jasper can be distinguished from chalcedony the determination of their crystallinity. with the help of a broad peak occurring between 497 and 452 cm"x. The IR-reflectance spectra of opals vary consider- Infrared reflectance spectroscopy is not as widely ably from those of quartz, chalcedony and jasper. used as infrared transmission spectroscopy (KBr- According to [2] the structure of opals varies from Pellet-Method). The main advantage of the former almost perfect crystalline opal-CT, to nearly amor- method lies in the fact that it is non-destructive and phous opal-A. Accordingly, opal-CT can be distin- does not require any special sample preparation. The guished from opal-A with the help of their characteris- only requirement is that the sample has a flat and tic spectra. The two main peaks for opal-CT are at or polished surface. The IR-reflectance spectrum of a near 1107 and 480 cm-1, while those for opal-A are sample is homologous to its absorption spectrum, shifted to 1116 and 470 cm"1, according to the degree with the exception that certain peaks are shifted of crystallinity. Furthermore, the two peaks at 780 and minutely to longer or shorter wavenumbers. 800 cm - \ which are typical of quartz, chalcedony and The IR-reflectance spectrum of a single crystal is jasper merge to a single broad peak in the case of dependent on its structural orientation. Therefore opals. several spectra have to be measured for the different IR-reflectance spectroscopy is also very suitable for orientations of the single crystal. According to [1], the investigation of silica glasses. According to [3], amorphous materials and glasses, and also fine glasses show infrared spectra whose pattern changes grained polycrystalline materials if their grain size is with composition and progressive ordering. The two small relative to the area being measured, do of course prominant peaks of quartz occurring at or near 1176 not show these differences. and 1109cm-1 merge to a single peak at about The present paper demonstrates the application of 1088 cm-1 in the case of silica glasses and obsidians. IR-reflectance spectroscopy for the identification of Simultaneously, the peak between 800 and 780 cm-1 the different types of microcrystalline and amorphous becomes even broader than it is in the case of opals. modifications of silica. Moreover, it is observed that As far as the structural similarity is concerned, the there is a correlation between the crystallinity and the different modifications of silica consist of Si04-tetra- shape and intensity of certain particular bands in the hedra which are linked together at their corners. IR-spectra of the samples. According to [4], the peaks between 800 cm-1 and The spectra were measured using an FTIR-Spec- 780 cm "1 represent the ordered or disordered arrange- tralphotometer (Perkin-Elmer, Model 1760), equipped ment of the Si04-tetrahedra in the different modifica- with an accessory for the measurement of reflectance. tions of silica. As it has been shown in [5], the crys- The sample area used was 3 mm in diameter. All sam- tallinity and also the ordered arrangement of the ples were ground and highly polished before measure- Si04-tetrahedra decreases from quartz to obsidian ment. (glass) as follows:

Reprint requests to Dr. A. Banerjee, Institut für Geowissen- quartz chalcedony opal-CT -»• opal-A schaften - Edelsteinforschung, Johannes-Gutenberg-Univer- sität Mainz, Mainz, FRG. -> obsidian (glass).

0932-0784 / 93 / 0500-0741 $ 01.30/0. - Please order a reprint rather than making your own copy. 742 Notizen

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WAVENUMBER (cm'1 ) Fig. 2. Shift of the major peaks between 1180 and 1088 cm ~1 from quartz to obsidian (glass) according to the change in the degree of crystallininity.

The shape and intensity of the two peaks at 800 and 780 cm-1 in the IR-reflectance spectra change gradu- OBSIDIAN ally from quartz to obsidian (glass) as the crystallinity decreases according to the sequence mentioned above (Figure 1). In addition to that, a systematic shift of peaks is noted between 1176 cm-1 and 1088 cm-1 from quartz to obsidian (glass) as the crystallinity of the samples gradually decreases (Figure 2).

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-1 WAVENUMBER ( cm') [1] R. J. P. Lyons and E. A. Burns, Economic Geology 58, 274 (1963). Fig. 1. Gradual changes of the shape and intensity of the [2] J. B. Jones and E. R. Segnit, J. Geol. Soc. Austral. 18, 57 peaks between 500 and 800 cm-1 in the sequence quartz- (1971). chalcedony-opal-obsidian (a natural glass). Note also that [3] R. J. P. Lyons, NASA-Technical Note D-1871 (1963). -1 the peak at 690 cm , which is quite prominent in the case [4] R. G. Milkey, Amer. Min. 45, 990 (1960). of quartz, becomes gradually broader from opal to obsidian. [5] I. I. Plyusnina, Sov. Phys. Dokl. Akad. Nauk. 24, 332 (1979).

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